1. Field of the Invention
The present invention relates to a sheet post-processing apparatus and an image forming system.
2. Description of the Related Art
For example, there is a known sheet post-processing apparatus that temporarily stacks, on a stack tray, sheets conveyed from an image forming apparatus, performs a binding process on a sheet bundle that is a stack of a predetermined number of sheets, and then discharges the sheet bundle.
A sheet conveyed onto the stack tray comes in contact with a roller member that rotates in a direction in which the sheet is fed back toward an upstream side in a conveying direction, so that the motion of the sheet is damped and the sheet falls onto the stack tray. Subsequently, the sheet is conveyed to the upstream side in the conveying direction on the stack tray by the roller member and a rear end of the sheet butts against a rear-end-position regulating member, so that the sheet is aligned in the conveying direction.
The rotation speed of the roller member is constant without a change between when the sheet falls and when the sheet is conveyed to the rear-end-position regulating member.
Japanese Patent Laid-open Publication No. 2009-227468 discloses a technology, in which the speed of a rotation member for conveying a sheet to a rear-end-position regulating member is reduced relative to the rotation speed of a roller member for causing a sheet to fall onto a stack tray, in order to prevent buckling that occurs when the sheet is excessively conveyed backward.
The state of a sheet that is falling onto a stacking surface of the stack tray is unstable in the air. Therefore, if a conveying force is applied to the sheet by the roller member, a landing position of the sheet fluctuates.
If the rear end of the landed sheet is too close to the rear-end-position regulating member, the sheet is excessively conveyed at the time of rear-end butting, and sheet buckling occurs.
In contrast, if the rear end of the landed sheet is too far from the rear-end-position regulating member, the rear end of the sheet may not be conveyed to the rear-end-position regulating member, and a binding failure may occur.
In Japanese Patent Laid-open Publication No. 2009-227468, the influence of the roller member while the sheet is falling onto the stack tray is not considered.
In view of the above circumstances, there is a need to provide a sheet post-processing apparatus capable of stabilizing a landing state of a sheet on a stack tray, and contributing to sheet alignment accuracy and post-processing accuracy.